Abstract

Long‐term, broadband measurements of transmission loss, TL, between a fixed source and a fixed receiver in littoral seas, permit isolation of the effects of bioacoustic absorptivity, αb, from other environmental effects on TL. Concurrent echo sounder measurements and biological sampling facilitate classification of the causes of observed absorption lines. Frequencies of maximum αb at night, when pelagic fish are predominantly dispersed near the surface, are consistent with a modified form of Minnaert’s equation for the resonance frequency of bubbles, f0, where the inputs are the effective radius and eccentricity of swim bladders. Frequencies of maximum αb during the day, when pelagic fish are predominantly in schools near the bottom, are downshifted, relative to Minnaert’s equation, due to the collective oscillations of schools (Diachok, 1999). The magnitude of αb at f0 (at night) is a function of the number density,n (number/m3), and other bioacoustic parameters. Nearly coincident absorption and echo sounder based estimates of n have been shown to be consistent (Diachok, Liorzou, and Scalabrin, 2000). The implications of these results for the design of the next generation of long‐term, broadband transmission loss experiments in littoral seas where n is high, such as the seas off California and China, will be reviewed.